Insulation Calculator
The R-value to aim for in your climate zone, how much to add over what is already there, and the bags of blown-in, packs of batts or sheets of foam board to buy for an attic, wall, floor or basement wall. It also counts the vent baffles, depth rulers and air-sealing supplies, and draws the insulation depth against the target.
Attic insulation reference
Attic floor, DOE
- Zone 1
- R-30 to R-49
- Zones 2–3
- R-30 to R-60
- Zone 4
- R-38 to R-60
- Zones 5–8
- R-49 to R-60
- Adding to 3–4″, zones 2–3
- R-25 to R-38
- Adding to 3–4″, zones 5–8
- R-38 to R-49
- IECC 2021, new homes
- R-30 / 49 / 60
Walls, floors, basements
- 2x4 wall cavity
- R-13 to R-15
- 2x6 wall cavity
- R-19 to R-21
- Floor, zones 1–2
- R-13
- Floor, zone 3
- R-19 to R-25
- Floor, zones 4–8
- R-25 to R-30
- Basement wall, zone 4
- R-10 foam or R-13
- Basement wall, zones 5–8
- R-15 foam or R-19
R-value per inch
- Blown cellulose
- R-3.5 to 3.8, settled
- Blown fiberglass
- R-2.2 to 2.9
- Fiberglass batts
- R-2.9 to 4.3
- Mineral wool batts
- R-4.0 to 4.3
- EPS foam
- R-3.6 to 4.2
- XPS foam
- R-5
- Polyiso, foil-faced
- R-5.6 to 6.5
Venting and details
- Attic vent area
- 1 ft² per 150 ft²
- Vents split high and low
- 1 ft² per 300 ft²
- Air space under baffle
- 1 in minimum
- Depth rulers
- 1 per 300 ft²
- Non-IC recessed lights
- 3 in clear
- Cellulose settling
- about 10–20%
- Attic hatch
- weatherstrip, foam lid
Safety Attics get dangerously hot: work in the morning, drink water and take breaks. Step only on joists or a board laid across them; a foot between the joists goes through the ceiling. Wear a P100 or N95 respirator, goggles and long sleeves. Keep insulation clear of non-IC recessed lights, flues and chimneys, do not bury live knob-and-tube wiring, and leave grey-brown pebbly vermiculite alone until it has been tested for asbestos.
Full notes
- Air seal before you insulate. Foam the holes around pipes, wires and flues, caulk the top plates to the drywall and weatherstrip the attic hatch: loose fill does not stop air, and air leaks carry more heat and moisture than the insulation can make up for.
- Put a vent baffle in every rafter bay that has a soffit vent, from the top plate up past the final insulation depth, so the blown fill cannot block the air path from the soffit to the ridge.
- Blown cellulose settles about 10–20% in its first year. Blow to the installed thickness on the bag chart, not the settled one, and staple a depth ruler about every 300 sq ft facing the hatch so you can see the level.
- Never compress batts: an R-19 batt squeezed into a 2x4 wall is worth about R-13. Cut them to the length of the cavity, split them around wires and pipes rather than stuffing them behind, and use unfaced batts when you add a layer over old insulation.
- Keep insulation 3 in away from recessed lights that are not IC-rated, and build a metal or noncombustible dam around chimneys and flues to the clearance on the flue label. Do not bury live knob-and-tube wiring.
- If the old attic fill is grey-brown and pebbly, it may be vermiculite that contains asbestos. Do not disturb it: have it tested or call a professional before you work in the attic.